This invention relates to a plasma enhanced chemical vapour deposition (PE-CVD) apparatus and to a method of performing PE-CVD.
PE-CVD is a well known technique for depositing various materials. It is well known to use PE-CVD in the production of semiconductor devices. In common with other methods of processing semiconductors, a very important factor in the realisation of a commercially useful process is the throughput of the system. A major problem which reduces the throughput is the clean process. The clean process is necessary to remove deposited material from the internal surfaces of the PE-CVD process chamber. Reductions in the time taken to perform a clean process or the time between clean processes will result in a higher throughput and a lower cost of ownership (COO). Modern clean processes have cleaning rates which are homogenous over the entire surfaces of the chamber. However, at least some PE-CVD processes result in depositions on the internal surfaces of the chamber which have an uneven distribution of deposition thicknesses. The deposition of silicon nitride, particularly at low deposition temperatures, provides an example of a PE-CVD process which produces a highly uneven thickness distribution of deposited material.
PE-CVD is commonly used to process silicon wafers. The typical PE-CVD single wafer chamber system design methodology is to limit the conduction of gas in the system. This is done with the intention of making the net pumping flow of the system across the wafer radial in direction. The intention is to provide uniform deposition across the wafer. Most commercial single wafer PE-CVD systems use ceramic spacers to tune the earth plane of the system, influence the shape of the plasma, and to achieve this radial conductance of gas. FIG. 1 shows an example of a prior art PE-CVD chamber, depicted generally at 10, comprising a chamber 12 having a platen 14 disposed therein. A “showerhead” 16, located at the top of the chamber 12, is used to introduce gases into the chamber 12. A plasma is formed using a plasma production device (not shown) as is well known in the art. The system 10 further comprises a lower ceramic spacer 18 and an upper ceramic spacer 20. The upper ceramic spacer 20 and the platen 14 define a relatively small first gap 22. The upper ceramic 20 and the lower ceramic 18 define a relatively small second gap 24 which leads to a circumferential pumping chamber 26. The circumferential pumping chamber 26 is in gas conducting communication with a pumping port. The pumping port is not shown in FIG. 1, but FIG. 2 does show the pumping port 28 and wafer entry slot 29. FIG. 2 is described in more detail below. The pumping port is connected to an exhaust line including a suitable pump in order to exhaust gas from the chamber to maintain a desired pressure within the chamber. Therefore, gases are exhausted from the chamber over a flow path which comprises the first pumping gap 22, the second pumping gap 24 and the circumferential pumping chamber 26. The first and second pumping gaps 22, 24 are relatively small in order to reduce the gas conductance of the system so as to produce a radial flow.